The invention relates to a drying section of a papermaking machine and more particularly to system for guiding a paper web to be dried, which travels together with a porous supporting felt between adjacent drying cylinders.
U.S. Pat. No. 4,677,762 discloses a guide or transfer roller that is provided with recesses and operates with an external vacuum box. The vacuum box is disposed to generate a vacuum along the inside of the felt to hold the web to be dried against the other side of the felt.
Patent publication WO 90/12151 discloses a guide box designed as a suction box. By means of a preferably mechanical air deflecting member, known per se from U.S. Pat. No. 4,856,205, the boundary air layer that would otherwise be carried by the felt into the pocket between the cylinders in the direction of the paper web transfer roller is deflected upward away from the pocket. Thereby the vacuum in the suction box can be kept low to save energy.
This known design provides an insufficient air seal into the pocket, so that some air is drawn between the outer wall of the suction box and the supporting felt as well as at the transfer roller, especially between the trailing gap and the suction chamber of the suction box. As a result, stagnant, ambient air is drawn in and the vacuum that is supposed to hold the paper web against the supporting felt is destroyed or rendered ineffective. Indeed, a positive pressure may even be generated.
In the system described in German publication DE-OS 4314475 for guiding a web to be dried, measures are taken to prevent the need from applying an internal vacuum at the interior of the transfer roller. Nevertheless, a powerful vacuum must still be generated in the circumferential grooves provided in the outer mantel of the roller by drawing a powerful vacuum from the external suction box.
To couple the vacuum box to the transfer roller two lengthwise sealing strips are provided. These sealing strips are mounted on the underside of the external suction box, each sealing strip projecting into the leading and trailing gaps, and nearly completely filling it. The strips also extend transversely with respect to the web travel direction over the entire length of the suction box.
Each of these two sealing strips, which are in the shape of relatively thick wedges, has a concave sealing surface that matches the arcuate shape of the roller mantel of the transfer roller. To avoid the risk that the sealing strips will exert a braking force on the rotating transfer roller, the lengthwise extending sealing strips must be installed at a considerable distance away from the transfer roller. The effect of the sealing strip is therefore partially lost. This risk also exists when the external suction box is not manufactured sufficiently precisely and/or when it flexes under its own weight or because of nonuniform heating.